Thermosetting polymers, such as polyurethanes, have many properties that qualify them as high performance polymeric materials, but there are still a few shortfalls. For example, polyurethanes still suffer from mechanical damage, such as when a hard or sharp object hits vehicle, it is likely that it will leave a scratch. To try to combat these types of mechanical damage to coatings, the automotive industry looks for coatings with high scratch resistance. Due to their hardness and elasticity polyurethanes exhibit good scratch resistance, but can still suffer from mechanical damage.
Mechanical damage occurs in all types of substrates. For example, to heal mechanical damages in plants, suberin, tannins, phenols, or nitric oxide are activated to prevent further lesions, whereas in a human skin outer flow of blood cells are arrested by the crosslink network of fibrin, giving rise to wound-healing. Concentration gradients or stratification in living organisms inspired the development of spatially heterogeneous remendable polymers, composites containing micro-encapsulated spheres, encapsulated fibers, reversible cross-linking, and microvascular networks.
Attempts have been made by others to repair mechanical damage to various substrates. One such example includes epoxy matrices containing a glass hollow fiber filled with a monomer and an initiator with the ‘bleeding’ ability to heal polymer networks during crack formation. Similar phenomenon was utilized in another approach, where a micro-encapsulated dicyclopentadiene monomer was introduced in a catalyst embedded polymer matrix, which healed the crack by the ring opening of the monomer. Reversibility of Diels-Alder reactions resulted in another attractive approach to thermally repair damaged areas which utilized malemide-furan adducts. Mimicking of microvascular structures, water-responsive expandable gels, and formation of supramolecular assemblies are other avenues of remendability.
While progress has been made in attempts to repair damage to various types of systems, coatings still lack the ability to repair mechanical damage to which they are exposed. A need exists for polymer systems that are capable of self-repairing mechanical damage to which they are exposed. It would be advantageous for the polymers to be useful in coatings, so that the coating could be able to mend itself. It would be further advantageous if the repair could occur when the coating is simply exposed to ambient conditions, such as UV exposure from the sun.